This invention relates to hydraulic systems and more particularly to an airless pump and an operating mechanism therefor, said pump being particularly useful in conjunction with the airless or hydraulic spraying of paint or other liquid coating materials.
In airless spray painting, the paint is supplied at high pressure in the order of about 1,000 to 3,000 p.s.i. to an atomizing nozzle having a small eliptically shaped orifice therethrough. The paint is atomized hydraulically upon passage through the orifice into a fan-shaped spray as is known in the art. In spray painting, it is conventional to spray intermittently, and a valve is associated with the nozzle to accommodate starting and stopping of the spray as desired or required.
Because of the necessity for intermittent operation, some means must be provided to control the internal pressure within the pump, to prevent excessive build up of pressure when the spray nozzle valve is closed. In order to overcome this problem, it has been proposed to use an air motor to actuate the pump, such motors being operable only when required to meet a pressure requirement in the pump. Air motors, however, have not fulfilled all of the requirements of the art since they require a source of air, which in the case of allegedly portable equipment requires bulky and expensive air compressors and pressure tanks. Many attempts have been made to employ an electric motor to drive high pressure pumps for airless spraying, but with little success.
In addition to preventing excessive pressure build up in an airless pump, it is also desirable to maintain a fairly constant pressure at the spray tip even though the spray gun may be operated intermittently. Otherwise, the paint that is initially emitted from the nozzle is not properly atomized. In order to achieve these and other desiderata of the art, various types of pressure by-pass arrangements have been employed in combination with an electric pump, one of which is shown and described in the Enssle, U.S. Pat. No. 3,433,415. There, a multiple valve arrangement is employed to maintain a constant pressure at the closed nozzle while allowing continuous operation of the pump and electric motor by providing a by-pass around the pump outlet when the nozzle is closed. Such arrangements require the use of many complex parts that are not ideally suited for changing of paint colors and cleaning, as well as service-free use. Moreover, airless pumps with electric motors have heretofore employed drive belts and pulleys between the motor and the pump that are subject to wear.